This invention relates to optical transmitters, and more particularly to an optical transmitter that integrates a laser head, optical modulator, and possibly a wavelength reference, within a common package to reduce insertion loss, provide greater output power over a greater dynamic range, and reduce overall system cost.
Some of the matter contained herein is disclosed and claimed in the commonly owned U.S. patent application Ser. No. 08/885,428, now U.S. Pat No. 5,982,964 entitled xe2x80x9cProcess For Fabrication And Independent Tuning Of Multiple Integrated Optical Directional Couplers On A Single Substratexe2x80x9d; U.S. patent application Ser. No. 08/885,449, now abandonded entitled xe2x80x9cMethod and Apparatus For Dynamically Equalizing Gain In An Optical Networkxe2x80x9d; U.S. patent application Ser. No. 08/885,427, now U.S Pat. No. 5,915,052 entitled xe2x80x9cLoop Status Monitor For Determining The Amplitude Of Component Signals Of A Multi-Wavelength Optical Beamxe2x80x9d and U.S. patent application Ser. No. 08/884,747, now U.S. Pat. No. 6,151,157 entitled xe2x80x9cDynamic Optical Amplifierxe2x80x9d all of which are incorporated herein by reference.
The low loss, light weight, small size, flexibility, and high intrinsic bandwidth of optical fiber make it a highly desirable medium for digital and analog signal transport. An optical transmitter generates a modulated optical signal which propagates through the optical fiber to a receiver end, wherein the optical beam is converted to an electrical signal. The optical beam may be modulated externally by an electrical signal representative of the information to be passed through the optical fiber.
Commercially available optical transmitters are made up of a plurality of discrete components interconnected by polarization-maintaining (PM) optical fiber. These components include a laser, an external optical modulator and control circuit modules. The packaging of a complete fiber-optic transmitter including these discrete components is relatively bulky and complicated. For example, currently available fiber optic transmitters produced for cable television (CATV) applications occupy a 19-inch rack drawer chassis, 3 inches or more high, housing power supplies, control circuits, laser, modulator, and amplifiers.
The potential military applications of RF and microwave fiber-optic transmitters are numerous. Possibly the largest military application is in the area of remotely mounted microwave antenna systems, such as phased-array antenna system designs, airborne radar warning-receiver direction-finding antenna systems, bi-static radar antenna systems, and many shipboard antenna systems. Practically any antenna system in which an RF or microwave signal is received or transmitted could benefit from direct microwave transport of the signal using fiber-optics between the antenna and the receiver/transmitter location. In most microwave antenna systems, a downconvertor/upconvertor system must be located in close proximity to the antenna aperture, due to the inefficiencies of metallic cables for transmission of microwave-frequency signals. The frequency converter electronics are therefore required to operate in the typically harsh environment of the antenna, which increases the size and cost of the front end packaging, and may limit the system desianer""s flexibility in antenna placement on the platform. Also, the downconvertor typically requires that a local-oscillator reference signal be distributed to the front end area.
If a miniature external modulator transmitter module was available that could provide an essentially xe2x80x9ctransparentxe2x80x9d microwave transport path over optical fiber, the frequency converter electronics could then be removed from the front end area, adjacent the antenna. This would not only reduce the size and complexity of the front end packaging, it would also improve overall system reliability, since fewer components would be located in the typically harsh front end environment. System performance also may actually be enhanced, since the frequency converter electronics typically limit the dynamic range of the downlink for most microwave systems. If the packaging and environmental constraints are relaxed on the downconvertor, enhanced dynamic range is more achievable.
An important application of the invention is telecommunications in which digital signals containing large volumes of voice, video, and data traffic are transmitted over optical fibers. At the higher data rates, the transmitter typically consists of a Distributed Feedback (DFB) laser and a modulator. Systems employing Dense Wavelength Division Multiplexing (DWDM) also typically contain a fiber coupler to tap off power and a wavelength reference, which is used in a feedback loop to stabilize the laser wavelength. The latter function is critical for DWDM where the optical signals from many transmitter are carried by a single optical fiber, yet can be separated from one another at the receive end because of the distinct wavelength used for each optical channel.
Currently, the optical transmitter""s components are assembled from separate packages, namely a standard DFB laser diode package and modulator package, with possibly an optical tap coupler and wavelength reference in two other packages, that are all coupled to each other with optical fiber. Significant coupling losses are incurred at the laser-fiber and modulator-fiber interfaces, because lasers and modulators support elliptic modes while fiber medium supports a circular mode. Moreover, fiber pigtails on the laser and modulator input have to be realized in polarization maintaining fiber, which adds cost to the packaging because it has to be precisely rotated. Elimination of the optical fiber interconnects between the components not only reduces optical losses but reduces transmitter cost associated with splicing and storing the fiber within the transmitter.
Other commercially-available optical transmitters include a laser assembly fixedly coupled to an optical modulator which are then rigidly mounted to a support bed. The purpose of fixedly coupling of the optical components is to insure precise alignment to thereby reduce the power loss resulting from misaligned optics. Alignment of the optical components of these transmitters is difficult and time-consuming which thereby, increase the costs of manufacturing.
In addition, these optical transmitters are sensitive to thermal changes as a result of the different coefficients of thermal expansion for the optical components. As the ambient temperature of the transmitter increases or decreases the varying amounts of thermal expansion of the components stresses the components, possibly altering their optical characteristics. The different coefficients of thermal expansion also may alter the alignment of the optical components and thereby negatively affect the optical beam emitted from the laser assembly. This is especially critical because the optical beam emitted from a laser diode is directly focused to the modulator. Any shift of the optical components greatly reduces the output power of the transmitter as a result of the misalignment of the components. Some prior art devices such as those marketed by the G.E.C. Marconi company are comprised of discrete components and include a thermocooler to help maintain temperature stability. However, these devices are not free from the aforementioned problems.
Furthermore the optical components are not replaceable or interchangeable because the components are mounted rigidly to each other and the support bed. If a component has failed or the wavelength of the optical beam wishes to be changed, the component cannot be easily removed or replaced without damage to the transmitter.
Accordingly, it is a principal object of this invention to provide an integrated optical transmitter that reduces insertion loss, provides greater output power over a greater dynamic range, and reduces cost related to assembly and interconnection of optical components.
It is another object of this invention to provide an integrated optical transmitter included within a single unit or housing.
It is a further object of this invention to provide a pre-aligned optical sub-assembly, which can be compliantly mounted to an optical bed, and which also has a surface to which a modulator can be fixedly secured.
It is a further object of this invention to provide an integrated optical transmitter that reduces misalignment due to varying coefficients of thermal expansion of the optical components.
It is yet another object of this invention to provide an integrated optical transmitter of the foregoing type having integrated wavelength control.
It is yet another function of this invention to provide an integrated optical transmitter wherein the optical components are interchangeable.
According to a preferred embodiment of the present invention, an integrated optical transmitter for use in an optical system includes an optical head assembly having an optical beam generator for providing an optical beam and a lens assembly collecting the optical beam and generating therefrom a formed optical beam. Also included is an optical modulator for receiving the formed optical beam for providing a modulated optical beam in response to received modulation signals. Interface optics are provided to receive the formed optical beam and to present the formed optical beam to the optical modulator. The interface optics provide optical coupling with the optical modulator to minimize insertion loss to the formed optical beam and to maintain a fixed optical relationship therewith
According to another aspect of the present invention, a method of fabricating an integrated optical transmitter includes the steps of:
(a) aligning optically a laser diode and an aspheric lens;
(b) securing the laser diode and the aspheric lens to a mounting element to define a laser head assembly;
(c) securing fixedly a focusing lens to the laser head assembly in optical alignment with the laser diode and aspheric lens;
(d) compliantly securing the laser head subassembly to an optical bed.
(e) securing fixedly an optical modulator to the focusing lens in optical alignment with the focusing lens.
According to yet another aspect of the present invention, a method of fabricating an integrated optical transmitter of the foregoing type also includes the step of controlling wavelength select control by means of a wavelength filter, such as a Fabry-Perot etalon, fiber Bragg grating, Michelson interferometer, or etalon with multi-layer dielectric films, which samples the light in the transmitter, and is included within a housing.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.